1. Field of the Invention
Exemplary aspects of the present invention generally relate to a rotary encoder, particularly to a device which measures the difference between the rotational center of a rotary disk and the center of a radial scale provided on the rotary disk to detect the rotational state of the rotary disk, i.e., the amount of rotational eccentricity.
2. Description of the Background Art
Devices that detect the rotational state of a rotary member, such as the rotation speed and the rotation amount (i.e., angle of rotation) thereof, include a photoelectric rotary encoder. In a photoelectric rotary encoder, the periphery of a rotary disk connected to a rotary shaft of the rotary member is provided with a radial scale including alternating light-transmitting portions and light-blocking portions, or light-reflecting portions and light-absorbing portions.
A light projector projects a light flux onto the radial scale, and light reflected by or transmitted through the radial scale is detected by a detection device. Then, using signals from the detection device, the rotational state of the rotary disk is detected.
In general, the detection accuracy of the above-described rotary encoder is significantly reduced if the center of rotation of the rotary disk and the center of the radial scale are not aligned, that is, if there is an eccentricity between the two centers.
Of course, the reduction in detection accuracy can be prevented by aligning the center of rotation of the rotary disk and the center of the radial scale, and to accomplish that the accuracy of manufacturing and assembly of the radial scale and the center of rotation of the rotary disk may be enhanced, as in certain known techniques. Alternatively, the radial scale may be attached to the rotary shaft while the center of the radial scale and the center of rotation of the rotary shaft are adjusted with the use of a microscope or the like, as in other background techniques.
If the manufacturing and assembly accuracy is increased, adjustment is unnecessary in the assembly or replacement of components, and thus the assembly work is simplified. However, to achieve such relatively high accuracy the cost is increased. Further, such relatively high accuracy is required not only of the manufacturing and assembly means, such as the methods and the tools employed, but also in a post-production testing and measurement.
Meanwhile, if the rotary disk is assembled after being properly aligned, the cost of manufacturing and assembling the rotary disk is not so high. However, it is difficult to assemble and adjust the rotary disk, and thus it takes a relatively long time for the assembly and adjustment.
FIG. 1 is a diagram illustrating an example of the post-production measurement of eccentricity of a rotary disk according to a background technique.
A rotary disk 100 illustrated in FIG. 1 includes a mounting portion 101 and a radial scale 102. The mounting portion 101, which is a hole near the center in FIG. 1, is used to attach the rotary disk 100 to a detected abject (i.e., rotary member) such as a roller.
To calculate the center of rotation c1 of the rotary disk 100, the coordinates of an arbitrary point p1 on the circumference of the mounting portion 101 are first measured at at least three locations (three locations in the drawing) with the use of a measuring microscope or the like. On the basis of the measured coordinates, the coordinates of the center of rotation c1 of the rotary disk 100 can be calculated.
Then, to calculate the center c2 of the radial scale 102, the coordinates of an arbitrary point p2 on the circumference of a circle pattern 103 drawn concentrically with the radial scale 102 are measured at at least three locations (three locations in the drawing). On the basis of the measured coordinates, the coordinates of the center c2 of the radial scale 102 can be calculated.
The previously calculated coordinates of the center of rotation c1 of the rotary disk 100 are then compared with the coordinates of the center c2 of the radial scale 102 to obtain an eccentricity amount D.
Ultimately, whether or not the obtained eccentricity amount D meets a separately set standard is checked, and whether or not the rotary disk 100 can be used as a product for a rotary encoder is determined.
In the above-described eccentricity measurement method, however, in order to perform accurate measurement, it is necessary to accurately select the respective measurement points p1 and p2 without misalignment. In order to accurately select the measurement points, the expertise of a measurer and complicated image processing by a measuring device are necessary. As a result, the time for completing the measurement is increased.
Further, in the measurement of the measurement points, which uses a projector or microscope, a pattern enlarged by the device is blurred. Therefore, it is difficult to accurately select the measurement points, and thus to accurately measure the eccentricity amount.